Metal-air cells have been recognized as a desirable means by which to power portable electronic equipment such as personal computers because such cells have a relatively high power output with relatively low weight as compared to other types of electrochemical cells. Metal-air cells utilize oxygen from the ambient air as a reactant in the electrochemical process rather than a heavier material such as a metal or metallic composition.
Metal-air cells use one or more air permeable cathodes separated from a metallic anode by an aqueous electrolyte. During the operation of a metal-air cell, such as a zinc-air cell, oxygen from the ambient air is converted at the cathode to hydroxide ions and zinc is oxidized at the anode and reacts with the hydroxide ions, such that water and electrons are released to provide electrical energy.
Recently, metal-air recharging technology has advanced to the point that metal-air cells are rechargeable and are useful for multiple discharge cycles. An electronically rechargeable metal-air cell is recharged by applying voltage between the anode and the cathode of the cell and reversing the electrochemical reaction. Oxygen is discharged back to the atmosphere through the air-permeable cathode and hydrogen is vented out of the cell.
Metal-air cells may be arranged in multiple cell battery packs to provide a sufficient amount of power output for devices such as computers. An example of a metal-air power supply is found in commonly owned U.S. Pat. No. 5,354,625 to Bentz et al., entitled Metal-Air Power Supply and Air Manager System, and Metal-Air Cell for Use Therein, the disclosure of which is incorporated herein by reference.
Attempts to increase even further the power output of metal-air cells have had mixed results. Increasing the power output of a cell usually involves operating the cell at a higher current drain. Such a higher load, however, can significantly decrease the total energy density of the system and greatly increase the production of heat, both of which are detrimental to efficiency and lifetime of the cell.
The composition of the air electrode or cathode is important in determining the power production capabilities and efficiencies of a metal-air cell. Air electrodes typically comprise carbon particles such as carbon black, an oxygen reduction catalyst, and a non-wetting binder such as tetrafluoroethylene. Secondary air electrodes also include an oxygen evolution catalyst. Oxygen reduction catalysts are also referred to as discharge catalysts and oxygen evolution catalysts are also referred to as recharge catalysts.
Suitable discharge catalysts include silver, cobalt oxides or spinels, transition metal macrocyclics, and perovskites. Suitable oxygen evolution catalysts include nickel, cobalt, iron, and tungsten compounds. These catalysts, with the exception of silver, are often added to the air electrode in the form of oxides in powder form. Generally described, an air electrode is formed by mixing carbon black, the catalyst powders and a non-wetting binder, forming this mixture into a sheet and adhering the sheet to a current collector.
Although the above described electrodes are effective in a metal-air cell, there remains a need for increased power output from a metal-air power supply without comprising the efficiency and lifetime of the cell.